This invention relates to apparatus and methods for tracking antenna position. It may be used for microwave, radar or other antennas, on test ranges or other locations.
Conventional synchro- and resolver-to-digital converters receive analog signals from a synchro or resolver and digitize those signals for output. Use of such converters for position measurement can result in group delay, however, comprising primarily time required for the digitizing process and propagation time delay for time of flight between the converter and the instrument to which its signals are supplied. Such propogation time delay becomes especially significant in remote position sampling applications. A conventional solution to minimize time required for the digitizing process has involved constantly supplying analog position signals to such digitizing converters.
Generally, antenna data is acquired by varying position, the independent variable, and measuring the dependent variables at defined position intervals. Many systems use record increments generated by constantly examining the output of a position indicator to determine whether the dependent variables are to be measured. The system controller must recognize the record increment, trigger the instruments, wait on the instruments to settle, gather the data, and acknowledge the record increment to the generating device. An overspeed condition exists when this sequence is not completed before the next record increment occurs.
To minimize system data acquisition time, the time between record increments must be minimized. The time between record increments may be varied by changing positioner speed; however, positioner speed is not usually the limiting factor. In a typical antenna measurement system, for instance, the time between record increments is limited by system controller processing time and record increment detection time. Record increment detection time is the time between the last record increment acknowledged and the next record increment and is affected by the position indicator's digital output update rate and group delay characteristics. The maximum variation in group delay plus the time until the next update is the minimum record increment detection time. A typical position indicator might have 8 ms group delay variation and a 10 ms update rate. For such a case, the minimum record increment detection time is 8-18 ms. If this record increment detection time could be eliminated, it could be subtracted from the time required between record increments increasing data acquisition speed.
Applicant's apparatus is designed to eliminate record increment detection time. Not only does the tracking algorithm provide high speed output update rates, but it also compensates for processing delays to give zero group delay variation at constant position or velocity. With an output update rate of 200 ns and a group delay variation of zero during a constant velocity scan, total record increment detection time is 200 ns. This feature will considerably decrease data acquisition time in many systems.
The present invention samples the antenna position at regular, but varying intervals and predicts the position of the antenna between such samples using an algorithm, unlike previous techniques. This predicted position may be provided for output at more frequent intervals than those at which antenna position is sampled, unlike other techniques which can provide output only as frequently as antenna position is input into the converter. The hardware of the present invention contains circuitry which utilizes velocity related input at predetermined intervals to calculate output position. Additionally, output may be captured to provide feedback for updating the algorithm.
Firmware of the present invention utilizes a tracking algorithm which receives two consecutive position input samples, notes the period of time between such samples and notes feedback information corresponding to the most recent position input sample to calculate the updated velocity to be applied to the position tracking hardware during the interval prior to reception of a new position input sample. When the new sample is received, the process is repeated.
Importantly, the apparatus and methods of the present invention are useful when input samples are taken at a location which is remote from applicant's position tracking hardware and firmware. In such a case, a synchronizing signal is sent over an asynchronous serial link to define the point in time at which the input sample is taken. The synchronizing signal insures that the feedback information received by applicant's apparatus corresponds with the point in time that the input sample is generated at the remote location.
Applicant's firmware accepts position input samples every 15-20 ms. It predicts the velocity over the interval between the current position sample and the next sample, and adds any necessary error correction. This velocity signal is then output to the hardware portion of the apparatus which comprises a counter chain whose value can be captured in a set of output latches. The counter chain is driven by a variable rate clock which, under microprocessor control, changes the value of the counter chain to correspond with the actual position of the axis. When the positioner is moving at a constant speed (such as during data acquisition on an antenna test range), the counter chain "moves" at the same speed, providing a real-time update to the position information. Since positioner motion is simulated, fast output update rates are obtained which are independent of conversion time and processor speed. However, output information from applicants, apparatus is nevertheless under microprocessor control which allows the flexibility of adding offsets to the angle of the position, switching the axis to be output or displayed, changing the range of an axis, or utilizing remote inputs. The preferred embodiment of applicant's position indication apparatus, sometimes herein referred to as the "1885", implements the tracking algorithm by using difference equations. The actual change in position is calculated by subtracting the last position sample from the newest position sample. The position error of the algorithm-simulated information is calculated by subtracting position feedback generated by the algorithm from the newest sample received by the apparatus. Adding these two terms, dividing by the increment of time between the last position sample and the newest position sample and multiplying by a constant determines the value of the velocity signal to be output to the hardware. The simulation circuit in the hardware integrates velocity to provide a position output. This output and the periodic position input are sampled at the same instant in time to determine the position error of the simulator which, at constant positioner velocity, becomes 0.
The equation for calculating new velocity to be applied to the hardware at time n is: EQU V.sub.n =K.sub.a *[(P.sub.a.sbsb.n -P.sub.a.sbsb.n-1)+(P.sub.a.sbsb.n -P.sub.f.sbsb.n)]/dt.sub.n-1.
where V.sub.n is the new velocity for time n to n+1, K.sub.a is a constant determined by the hardware, P.sub.a.sbsb.n is the newest actual position sample taken at time n, P.sub.a.sbsb.n-1 is the last actual position sample taken at time n-1, P.sub.f.sbsb.n is the feedback sample at time n and dt.sub.n-1 is the length of time from n-1 to n.
The hardware provides the position feedback term according to the following formula: EQU P.sub.f.sbsb.n =P.sub.f.sbsb.n-1 +(dt.sub.n-1 *V.sub.n-1 *K.sub.b)
where P.sub.f.sbsb.n-1 is the position feedback sample at time n-1, Vn-1 is the velocity applied from time n-1 to n and Kb is a constant determined by the hardware.
In addition, the hardware provides a position output which is available at any time (within 200 ns) between sample points described by the following equation: EQU P.sub.o.sbsb.n (t)=P.sub.f.sbsb.n +[K.sub.b *V.sub.n *(t-t.sub.n)],
where P.sub.o.sbsb.n (t) is the position output as a function of time for the period from time n to time n+1 and the term t-t.sub.n is the time elapsed since time n (in 200 ns increments).
Applicants' preferred embodiment of a position data processor according to the present invention, sometimes referred to herein as the "1886", multiplexes three axes of input data and transmits serial data to the 1885 Position Indicator. The serial interface, selectable as either RS-232C or RS-449, may be connected to any communications device which can accept one of these formats. Fiber optic links, modems, microwave links, and direct cable connection are all examples of how the 1885 and 1886 might communicate with each other to provide a means for obtaining position information from a remote site.
Applicants' 1886 Position Data Processor also has an auxiliary serial port which can be configured to provide ASCII position information suitable for input to a portable terminal. Such a terminal may, for instance, be carried to a remote position or by service personnel to give an on-site readout of position.
It is therefore an object of the present invention to provide position indication apparatus and methods which allow information output at more frequent intervals than those at which information from the positioning device is sampled.
It is an additional object of the present invention to provide position indication apparatus and methods which minimize group delay and thus provide more accurate information.
It is an additional object of the present invention to provide position indication apparatus and methods which reduce the complexity of position measurement hardware, which do not require continual input for reduction of group delay, and for which group delay is negligible when the positioner is at constant velocity.
It is a further object of the present invention to provide position indication apparatus and methods which accommodate remote positioner information sampling with minimum group delay.
It is a further object of the present invention to provide position indication apparatus and methods with information input sampling circuitry to accommodate multiplexed inputs.
It is a further object of the present invention to provide position indication apparatus and methods for increased flexibility, including adding offsets to positioner angle, switching the positioner axis to be output or displayed, changing the range of a position or axis, and accommodating remote input.
Other objects, features and advantages of the present invention will become apparent with reference to the remainder of the specification, drawings and claims herein.